Fix implementation of filterKeys

Fix implementation of `IterableView#filterKeys` and
add a commented-out, fixed implementation of
`IterableView#mapValues`.

Author: NthPortal

compat/src/main/scala-2.11_2.12/scala/collection/compat/PackageShared.scala

@@ -284,7 +284,14 @@ class MapViewExtensionMethods[K, V, C <: scala.collection.Map[K, V]](
       implicit bf: CanBuildFrom[IterableView[(K, V), C], (K, W), That]): That =
     self.map[(K, W), That] { case (k, v) => (k, f(v)) }
 
-  def filterKeys[That](p: K => Boolean)(
-      implicit bf: CanBuildFrom[IterableView[(K, V), C], (K, V), That]): That =
-    self.collect[(K, V), That] { case (k, v) if p(k) => (k, v) }
+  // TODO: replace the current implementation of `mapValues` with this
+  //       after major version bump when bincompat can be broken
+  /*
+  def mapValues[W](f: V => W): IterableView[(K, W), C] =
+    // the implementation of `self.map` also casts the result
+    self.map({ case (k, v) => (k, f(v)) }).asInstanceOf[IterableView[(K, W), C]]
+   */
+
+  def filterKeys(p: K => Boolean): IterableView[(K, V), C] =
+    self.filter { case (k, _) => p(k) }
 }

compat/src/test/scala/test/scala/collection/ViewTest.scala

@@ -35,4 +35,12 @@ class ViewTest {
     assertEquals(oldStyle.toMap, newStyle.toMap)
   }
 
+  @Test
+  def filterKeysMapValues(): Unit = {
+    val m = Map("a" -> 1, "b" -> 2, "c" -> 3)
+    assertEquals(Map(), m.view.filterKeys(_.length > 1).mapValues(_ + 1).toMap)
+    // TODO: uncomment after major version bump when `mapValues` can be fixed
+    //assertEquals(Map(), m.view.mapValues(_ + 1).filterKeys(_.length > 1).toMap)
+  }
+
 }